Evaluation and Design an Efficient Mobile WiMax Architecture Using Coded OFDM System
Shankar ( (Asst.Prof)Abstract— Although the increasing high speed requirement of WiMAX network world wide its performance is required to evaluate for optimum solutions The increasing requirements of high speed WiMAX network over worldwide crates need to evaluate its performance for optimum solutions. The convolution coded OFDM systems are used for the improved BER performance but it is always required to find the best OFDM parameters for optimum system performance. This dissertation evaluates the performance of the Trellis coded WiMAX OFDM system over the AWGN channel for Mobile communicationapplication. In this dissertationOFDMsystem is modeled by using the different Trellis coded structures with variable constrain length for ½ code rate. As the FFT size affects the OFDM system design significantly. In this dissertation the performance of different Trellis structures is evaluated for coded interleavedOFDMsystemforvariableFFT size along with the different modulation orders viz. BPSK, QPSK, and 4QAM.
Key words: WiMAX, OFDM, Wireless Communication, AWGN,Interleaving,TrellisCoding
1. INTRODUCTION
WiMAX have become a popular standard and choice of communication system designers from last two decades. Early days of communication systems were basically dependentonanalogsolutions.Thesourcesofinformation incommunicationsystemsarealsoanaloginnature.Weare now leaving in an era of a digital world. The numbers of users for communication systems are exponentially increasing and demand of higher processing speed of wireless communication systems have also significantly increased.Wehavereachedtogigabitsofdatatransmission capability till now. Many wireless communication applications have became part of human’s life including. ShortMessageServices(SMS),broadbanddatatransmission, broadband video streaming, high speeds file transferring, video and call conferencing etc.Data transmission using broadband Internet on Wireless Local Area Network (WLAN) with wider area network etc is an example and common uses of high speed wireless communication systems.Thebroadbandutilityandrequirementisfurther extended by smoothly transmitting the multimedia data wirelesslybytheengineersall aroundtheglobe.Thisgives risetothenewconceptofhighspeedwirelessbroadband communicationitisrequiretodesignancheaperandflexible system in worst environment. The Orthogonal Frequency
DivisionMultiplexing(OFDM)istheusedsolutionatphysical layerofthesehighspeedwirelessapplications
WiMAXisawirelesstechnologybasedondefinedstandards whichprovideshigherthroughputbroadbandconnections over the long distance communication links. WiMAX technologyisusedforavarioustypesofapplications,which includeswirelessbroadband
Connections, mobile hotspots uses and high speed data connectivity
TheMobileWiMAXusestheIEEE802.16estandards[10]for designingthephysicallayerstandardsofwirelessMAN.Most commonly OFDM is a standard modulation technique or multiplexing used for physical layer or MAC layer of MAN networks.MostoftheWiMAXsystemsarebasedontheFFT sizeof256OFDMstandards.
CommonInternettrafficusesthe16FFT OFDMscheme.A basicarchitectureofthephysicallayeroftheMobileWiMAX isshownintheFigure2.
Jagdish D. Kene et al [1] have presented a performance evaluation of the coded OFDM system, for the WiMAX application,theyhavecomparedtheperformanceoftheBER forthedifferentkindofthedecoderarchitecture.Yogendra Pateletal[2]havepresentedtheperformanceenhancement of WiMAX OFDM and presented the BER comparison. Mohammed Aboud et al. [6] have implemented the OFDM basedWIMAXsystemforMulticoreSoftwaredefinedradio system. Sweety et al. [13] has presented an performance evaluation of the OFDM system for WiMAX for different modulationtypes.Theyhaveconcludedthatundertheless distorted channel the QAM can be used and for distorted channeltheQPSKcanbeopted.
OFDMiswidelyusedtechnologyforWiMAXimplementation [6].OFDMisusedasmultiplexingtechniqueormodulation technique which supports the higher channel utilization. Thereforeitisrequiredtodesignefficientcodingmethodby evaluating the parametric performance for WiMAX OFDM whichisthemainfocusofthepaper.
The OFDM signal transmission technique contains the followingmajoradvantages:
It offers the efficient use of the available bandwidth
OFDMspectrumbecome
more resistant to the frequency selective fading due to orthogonality compared to single carrier communicationsystems
OFDMiscapableofminimizingtheISIandIFIbyusingthe insertion oftheadditionalcyclicprefix.
Usingprechannelcodingandinterleavingcancompensate forchanneldistortions.
OFDMiscomputationallyefficientbyusingFFTtechniques toimplementthemodulationanddemodulationfunctions
NoneedtoimplementachannelestimatorinOFDM.
OFDMislesssensitivetosampletimingoffsetsascompared tosinglecarriersystems.
It provides excellent protection against co channel interferenceandparasiticimpulsivenoise.
1) TheOFDMsignalhasanoiselikeamplitudewitha verylargedynamicrange;thereforeitrequiresRF poweramplifierswithahighpeaktoaveragepower ratio
2) Itismoresensitivetocarrierfrequencyoffsetand driftthansinglecarriersystemsareduetoleakage oftheDFT.
2. CHANNEL CODING TECHNIQUES
ForimprovingtheefficiencyofOFDMsystemntermsofBER various pre channel encoding architecture have been designed. Commonly used channel coding methods are as Convolutionalfollows:
Coding:
Convolutional codes are represented by the three main parametersandsizeoftheblockcodeisrepresentedWhere; n=thenumberofoutputbits.k=thenumberofinputbits m=numberofthememorycomponentTheratiooftheinput bitstotheoutputbitsisgivenbycoderate.
The code rate is the one of the measure of the coding efficiency.Mostcommonlytherangeofinputandoutputbits (kandn)rangesfrom1 8.Thenumberofmemoryelements ��rangesfrom210.
Constrain Length: It is another design parameter abbreviated
The main drawbacksof the OFDMsystemcan belisted as Wherefollows;L
istheconstrainlengthisalsorepresentedbyLor TheK.
constraintlengthLrepresentsthenumberofbitsinthe encoder memory that directly affects the generation of output nbits.insomeliteraturestheconstrainlengthL is representedbyapproximateproductofthe����.anexample oftheconvolutionalencoderwiththeconstrainlength3,and thecoderateof1/3isshownintheFigurebelow.
ConvolutionalencoderforOFDM
Trellis Encoder: Trellis encoder uses two or more Convolutionalcoderconnectedinparallelwithinterleaver betweenthem.InTrelliscoderbasedconvolutionalcodeis convertedinto
spectral representation of multiple sub carriers is then transformedintoequivalenttimedomaindatausinganIFFT blockwhichiscomputationallyveryefficient.Atthereceiver the FFT block performs the vice versa. The procedure is equivalent to conversion of frequency domain to time domaintofrequencydomain
4. RESULTS AND DISCUSSIONS
In the current work it has proposed to design the system with Trellis coded bit interleaved system of COFDM. It is concludedthatusingprecodingmayreduceBERandusing bit interleaving reduces burst errors and makes system robustagainstchannelnoise.Followingmajorconclusions aredrawnexperimentally
IthasbeenconcludedthatdesigningtheOFDMsystemwith minimumFFTsizemayminimizetheBERbutatthecostof smallerSNRperformance.
Figure4AnTrellistreeandStatediagram
It is also concluded that as the NFFT size is increased the SNR performance of the system for different modulation techniques are increased significantly it is because more datacanbetransmittedwithhighercapacity.
Therefore it has become necessary to carefully adopt the propercodingandmodulationmethodathigherSNRvalue While higher SNR and BER performance is achieved by 4QAMwiththemorepayloadcapacityalso.
It is found that using the Trellis Encoder significantly improvedtheBERperformanceofthesystem
In the dissertation different Trellis structures are implementedforcoderateof½byincreasingtheconstrain lengthfrom3to10.
It is found that at SNR less than 12 the constrain length 3 givesminimumBERandathigherSNRgreaterthan12the constrainlength7Trellisstructureisperformingbetterin termsoftheBER
SimplifiedblockdiagramofaprocedureoftheOFDMsystem isshownintheFigure5.Itcanbeobservedthatefficiencyof theOFDMishighlydependentonIFFTandFFTefficiency. The M QAM modulation is implemented on the input bit streambeforebeingtransmittingtheinformationbitonthe selectedchannelthroughanOFDMtransmitter.Thedigital data to be transmitted is converted into a mapped sub carrier’samplitudesandphasesbyusingQAMmodulation techniques at the transmitter section. After data is modulated a serial to parallel convertor block converts it into N parallel multiple sub carrier streams. Orthogonal
5. CONCLUSIONS
methodsperformancetheEncoderpayloadandBEROFDMagainstBERcodedreductionthereforedependsmobileVideonumberWIMAXcommunicationThispaperhasfocusedtodesigningofOFDMbasedWiMAXsystems.TheOFDMiswidelyoptedwirelessschemeandattractedmanyresearchers.OFDMhasofapplicationsViz.BroadbandInternet,DigitalBroadcasting,AudioBroadcasting,WiFi,andinWiMAXapplications.TheefficiencyofOFDMontheefficiencyofthechannelperformancemanytechniquesaredesignedforchannelfadingandforcoding.IthasproposedtodesignthesystemwithTrellisbitinterleavedsystem.UsingprecodingmayreduceandusingbitinterleavingmakessystemrobustnoiseIthasbeenconcludedthatdesigningthesystemwithminimumFFTsizemayminimizethebutatsmallerSNRperformance.WhilehigherSNRBERperformanceisachievedby4QAMwiththemorecapacityalso.ItisfoundthatusingtheTrellissignificantlyimprovedtheBERperformanceofsystem.Infutureisalsorequiredevaluatingtheofthesymbolerrorratesandinterleavingfordifferentsymbolratesandsizes.
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